This is a huge issue for all of us who really care how we are perceived by our colleagues in the Windows world. As of the time of this post, I've found 3 ways to address this, none of which are ideal.
The core issue is that OS X Mail's "default message font" controls only the display of the font locally. It does not send the font's specifications into the outgoing email. As a result, any client (Outlook included) will simply display your message in its default reader font: Times New Roman.
To solve the issue you have to explicitly force a font "change" into your message so OS X Mail will send font-formatting instructions within your Rich Text email. I've found these as the three ways to do this:
- Use your Signature to Set the Font - Set your signature to anything you like, but add a couple of extra lines above it and set those lines to the font you want. Apple Mail still puts a single line at the top of your message, but as long as you start typing on the SECOND line and delete that first line, you'll get the font you specify into the outgoing message.
- Plug In Software - For $4.99 in the Apple Store there is an app called MessageFont (also see messagefont.com) that changes the font for you automatically to any font you want. Unfortunately, it works by overriding keyboard shortcuts for new messages (which is probably the best it can do given the limitations in Apple Mail APIs). I did not try it because I thought always using the keyboard shortcut was as much trouble as remembering to remove the top-line of my message. It is another option, though.
- Manually Set the Font - Click the "A" icon at the top of the compose-message window to always show your "format bar" and simply manually set the font on the text you send with every email. This is highly manual and seems the worst of the 3 options, but is the most straightforward and easiest to explain.
The secondary issue is that Windows/Outlook will STILL not map your fonts unless the font-face name is identical, and even when identical, will map font sizes differently than you expect. A previous poster has provided size mappings. My own experimentation shows that your best bet at a good looking / reasonably sized font on both systems is: Arial 11 or 12 point as set on the Mac. This results in a readable and similarly sized font on both the Windows and OS X side. I use Arial 11 for my signature, and the slightly larger Arial 12 for my message text.
Hope all this info helps.